In the past, there have been many attempts to develop a system for measuring hardness which would not require the observation/measuring of the indentation produced by an indenting tip. In the publication of the International Patent application No. WO-A-86/06833, a method is described for determining hardness by measuring the electrical contact resistance between the indenting tip of a hard and semiconducting material and a test specimen. This method requires the measurement of a contact resistance after having created an electrical discharge between the indenting tip and the metallic test specimen. The most suitable material indicated for this semiconducting indenting tip is silicon carbide. However, this method is not fully reliable due to the variability of tip material resistivity to changes in temperature. Furthermore, it is strongly affected by the contact conditions between the test specimen and the indenting tip. In a successive publication, WO-A-88/03644, a new device and method aimed at overcoming the problems denounced by the method of the earlier publication were illustrated. In this new method only a band, or a geometrically defined sector of the indenting tip surface was made conductive, either by depositing a conducting film, or by implanting specific ionic impurities into the tip material defined areas of the surface. The hardness was substantially determined as a function of the required load necessary to make the indenting tip penetrate the test specimen deep enough to establish an electrical contact with the conducting parts of the tip surface, connected to a measuring circuit. The defined areas of the tip surface were made conductive either by implanting impurities into them, or by depositing a conducting film on these specific areas, as for example, a titanium nitride film. This hardness measuring method is of the "run-length limited" type. In this method, the moment for reading the load applied to the indenter is set when an electrical continuity between the test specimen and the indenter occurs. However, the plastic deformation characteristics of the material affect the flowing of the plastically deformed material along the indenting tip sides introducing errors which are difficult to control. Therefore, even a small inclination of the incidence axis of the indenting tip with respect to the test surface can alter the measurement.
An intrinsically more reliable method for determining hardness, through the measurement of contact resistance, was put forward in an earlier Italian Patent Application No. 83639 A/90 filed on the 24th Oct. 1990 by the present applicant himself (which then brought about the International Publication No. WO 92/08119). According to that method, the hardness of a metallic piece is determined on the basis of the ratio between the contact resistance on a sample of known hardness characteristics, and that on the test specimen, at one, or preferably more, levels of loading. The indenting tip may be a IIp diamond, or other material of adequate electrical conductivity. In the corresponding International Patent Application (WO 92/08119), the use of a diamond tip preimplanted (bombarded) with accelerated nitrogen ions for creating a superficial film or "skin" of adequate conductivity, is described.
The practice of making the measurements after the application of an oil film onto the metallic surface, and the peculiar method for determining hardness--by comparing the readings on a standard material with that on the test specimen--provides an acceptable rate of reproducibility and reliability of the measurements. Nevertheless, the electrical conductivity of a superficial film implanted on a diamond crystal strongly depends on temperature. This remains a problem which is particularly felt in the case of a hardness tester to be used "in the field".